Method of producing polyimide matrix composite parts

ABSTRACT

The invention relates to a method of producing rectifier parts which are made from a laminate composite material comprising reinforcing fibres that are embedded in a heat-polymerised polyimide resin matrix. The aforementioned parts comprise an inner platform, an outer platform and at least one solid blade which connects said platforms. The invention is characterised in that it comprises the following steps consisting in: a) producing the essential parts of the platforms, blades and the blade/platform connecting zones as separate structural elements, by stacking or winding layers of reinforcing fibres which have been impregnated with resin and which are used to form said structural elements, with the exception of the outer layers which form at least the boundary wall of the gas stream through the rectifier; b) imidizing separate structural elements; c) assembling said separate imidized structural elements; d) adding the outer layers of reinforcing fibres which have been impregnated with resin in order to form the part; e) placing the part thus produced in a mould/counter-mould employing compression polymerisation; and f) polymerising the part by subjecting same to compressive stresses.

The invention relates to the field of parts made of ahigh-temperature-resistant composite, such as the vanes of an inletguide vane assembly on a turbojet, comprising reinforcing fibersembedded in a heat-curable resin matrix, said parts being able to havean outer platform, an inner platform and at least one full bladeconnecting said platforms.

Composites based on carbon fibers and a heat-curable resin matrix arebeing increasingly used for producing parts located in the cold regionsof aviation turbomachines, especially those for military use. This isbecause such composites exhibit excellent mechanical properties and havea much lower density than that of the metal alloys normally used.

The choice of resin type is made in particular according to thetemperature to which the part will be subjected under the normaloperating conditions of the turbomachine.

The known RTM (resin transfer molding) process consists in placing thereinforcing fibers in a mold having the shape of the finished part, ininjecting liquid resin under low pressure into the mold and in curingthe resin maintained under pressure. This process allows a variety ofdifferent parts to be obtained. However, the organic resins employedwith this process are of the epoxy/bismaleimide type and do not have asufficient temperature resistance for application to these turbomachineparts.

Moreover, organic resins resistant to higher temperatures exhibitinsufficient flow before curing.

Therefore either sheets of fabric or unidirectional sheets, that is tosay sheets in which the fibers are parallel and held in place relativeto one another by the resin, are used to produce the part by drapemolding, these fabrics or these fiber sheets being pre-impregnated witha resin resistant to higher temperatures, on specific tools, and thecuring operation is carried out in a mold allowing the part to becompressed.

French Application filed under No. FR 97/02 663 proposes a method forproducing thin-walled hollow parts made of a laminated composite basedon a PMR 15 heat-curable polyimide resin, PMR 15 being a trademark ofthe company Cytec. This method consists in producing a siliconeelastomer core which has the shape of the cavity of the part and can bedestroyed at high temperature, in draping the core with at least onelayer of prepreg fibers, in placing the core/prepreg-fiber assembly in amold, in curing the resin under compression and then in demolding andremoving the core. The mold comprises a female portion that supports oneof the walls of the part and a male portion that supports the other walland is capable of sliding in the female portion during curing. Thecompression exerted by the mold during curing, combined with the thermalexpansion of the silicone elastomer core, allows the composite to flowin order to reduce the porosity that results from air bubbles trappedbetween the fiber plies and also the gaseous emissions from the resinduring curing, thereby expelling the excess resin and increasing thefiber density.

This method described in the above document applies to thin-walledhollow parts.

As regards the production of engine casings made of a laminatedcomposite, which have an outer platform, an inner platform and at leastone full blade that joins said platforms, by curing in a single cycle,the gaseous emissions from the resin during the curing cycle would betoo great to allow a satisfactory level of porosity to be achieved inthe walls, owing to the large thickness of the walls constituting theblades and the platforms. This would in particular result in a poorsurface finish, including that of the walls that define the stream ofgas flowing through the guide vanes. Now, the surface finish is a factorthat influences the efficiency of the guide vanes, and beyond, ofturbomachines.

The amount of volatiles given off depends on the polyimide chosen. Now,for reasons of safety of operators working in manufacturing shops, it isa question of prohibiting the use of PMR 15 resin and of replacing itwith other polyimides such as Avimid-R, Avimid-R being a trademark ofCytec, which give off even more volatiles than PMR 15 during curing.

The object of the invention is to propose a method for producing thevanes of inlet guide vane assemblies made of a polyimide-matrixcomposite, the walls of which, defining in particular the gas stream,exhibit a good surface finish.

The invention is based on the fact that most of the volatile gases aregiven off during the first temperature-rise phase of the curing cycle,during which phase the monomers combine to form small and mutuallyindependent systems (dimers and trimers), the formation of long andmutually crosslinked chains taking place during the followingtemperature-rise phase, which is accompanied by volatile gases beinggiven off.

The method according to the invention is characterized by the followingsteps :

-   -   a) the essential portions of the platforms, of the blades and of        the blade/platform connection regions are produced, as separate        structural elements, by superposition or winding of the layers        of prepreg fibers (reinforcing fibers preimpregnated with resin)        that will constitute said structural elements with the exception        of the external layers that have to form at least the boundary        wall for the stream of gases flowing through the guide vanes;    -   b) said separate structural elements are imidized;    -   c) said separate imidized structural elements are assembled;    -   d) the external layers of prepreg fibers are added in order to        form the part;    -   e) the part thus formed is placed in a compression curing        mold/contermold assembly; and    -   f) the part is cured by subjecting it to compressive forces.

The term “imidized” is understood to mean that the separate structuralelements are prepolymerized, that is to say said elements are heatedaccording to an established temperature-rise law so as to fix themonomer system without creating covalent chains. During this imidizationoperation, most of the volatile gases are eliminated.

During step b), the elements to be imidized are placed in suitablemolds, which molds allow the gases to be removed, allow the excess resinto flow out, and give the imidized parts their final form.

At the end of the imidization operation, the imidized structuralelements may have an imperfect surface finish, which will be rectifiedduring the curing step.

This is because the imidization step gives imidized elements havingareas starved of resin essentially on the surface. The addition andcuring of the nonimidized prepreg layers makes it possible to moderate,or even eliminate, these flaws.

During the operation of curing the imidized elements that have beenassembled and covered with fresh prepreg layers of small thicknesscompared with the total thickness of the imidized elements, the volatilegases emanate from these nonimidized prepreg layers or are eliminatedduring the first temperature-rise phase, the excess resin from theexternal layers fills the pores of the preimidized structural elementswhich no longer give off any volatiles. During the secondtemperature-rise phase, which consists in the actual curing of theentire part, that is to say the creation of covalent chains throughoutthe part, there are almost no more volatile gases given off, and thepart is subjected to a high compression. This causes the resin to flowand results in a surface finish that conforms to the external layersadded to the imidized structural elements.

Advantageously, the external layers of fresh prepregs are added bydraping the pressure side of a blade, and the adjacent portions of theinternal faces of the platforms, with first joining prepreg layers andby draping the suction side of a blade, and the adjacent portions of theinternal faces of the platforms, with second joining prepreg layers.

Thus, after curing under compression, these prepreg layers form pliesexhibit excellent surface finish and greatly stiffening the partproduced.

Other advantages and features of the invention will emerge on readingthe following description given by way of example and with reference tothe appended drawings in which:

FIG. 1 is a cross section through a sector of an inlet guide vaneassembly in a plane containing the axis of a turbomachine;

FIG. 2 is a front view of the sector of FIG. 1;

FIGS. 3 and 4 show, respectively, the tools and the stacking of prepreglayers in order to produce the essential structural portions of theinner and outer platforms;

FIG. 5 shows the production of a blade/platform join using a strip ofprepreg;

FIG. 6 shows the tools used to imidize the blade/platform joins;

FIG. 7 is a cross section on the line VII-VII of FIG. 6;

FIG. 8 shows the arrangement of the various tools in a bladder made ofpolyimide film, especially “Kapton” film, “Kapton” being a trademark ofDuPont;

FIG. 9 shows the temperature curve during the imidization operation;

FIG. 10 shows the temperature curves during the curing operation; and

FIG. 11 shows the layers of fresh material to be added to the pressureand suction sides of the blades.

FIGS. 1 and 2 show a one-piece compressor guide vane sector 1 of anaviation turbomachine, made of a laminated composite comprisingreinforcing fibers embedded in a heat-cured resin.

This section 1 has two blades 2 a and 2 b that extend radially betweenan inner platform 3 and an outer platform 4. The upstream edge 4 a anddownstream edge 4 b of the outer platform 4 have a smaller thicknessthan the central portion of this platform 4.

The upstream edge 3 a and downstream edge 3 b of the inner platform 3have a smaller radial dimension than that of the central portion.

The references 6 a and 6 b denote the regions where the blades 2 a, 2 bare joined to the inner and outer platforms 3, 4.

According to the present invention, the sector 1 is made from severalseparate structural elements by superposition or winding ofunidirectional plies of fabrics of reinforcing fibers, especially carbonfibers, pre-impregnated with a heat-curable organic resin, especially apolyimide resin resistant to temperatures of around 300° C.

The outer platform 4 is thus produced from a structural element with thereference 40 in FIG. 1.

The inner platform 3 is produced from a structural element with thereference 30.

The main portions 30 and 40 of the inner and outer platforms 3 and 4 areproduced in a conventional manner by the stacking, pressing and bondingof parts precut from a roll of fabric or sheet preimpregnated withresin, on the tools with the references 35 and 43 in FIGS. 3 and 4.Several prepreg fabric pieces form the structural element 30 and thestructural element 40. The number of fabric pieces forming eachstructural element 30 or 40 is chosen according to the strength andthickness desired. The reinforcing fibers of each fabric piece areplaced either along the axial direction, or in the circumferentialdirection, or at 450 to them, the directions of the fibers beingspecified in a precise operating instruction.

The tools with the references 35 and 43 in FIGS. 3 and 4 are male partsof imidization molds, the female parts of which are not shown in thedrawings.

The blade/platform join regions, with the references 6 a, 6 b in FIG. 2,on the pressure side and suction side, are also made in the form ofseparate structural elements. As shown in FIG. 5, each of these joinregions is produced by rolling a strip of prepreg fabric in thedirection transverse to the direction of the fibers, this strip beingplaced in the impression of a suitable mold 70 visible in FIGS. 9 and10, and having a cover 71.

The core of the blades 2 a, 2 b is also produced by stacking strips offresh prepreg fabric or sheet and is placed between a pressure side tool80 and a suction side tool 81, so as to constitute separate structuralelements as shown in FIG. 2.

The separate structural elements 30, 40, the blade/platform join regions6 a, 6 b and the core of the blades 2 a and 2 b are placed tightly intheir tools, as was described above. These tools include felts forextracting the volatile gases and for flow of the resin. All the toolsare placed in one and the same polyimide bladder, such as a “Kapton 82”film, “Kapton” being a trademark of DuPont, and then all the separatestructural elements constituting a sector 1 are imidized in anautoclave.

The autoclave imidization cycle depends on the type of resin used.

For polyimides, the temperature is raised, for example, from 20° C. to250° C. at a rate of 0.5° C./min and then maintained at a temperature of250° C. for a period of 120 minutes. After cooling at a rate of 1°C./min, the parts are demolded when the temperature reaches 40° C. Arelative vacuum of −50 mbar is maintained in the bladder throughout theduration of the imidization cycle. FIG. 9 shows the temperature curve asa function of time.

During the imidization cycle, the monomer system is converted inparticular into a system of dimers and trimers. About 80% of thevolatile gases are removed during this step.

When the separate structural elements have been immiodized and demolded,they are assembled in a specific tool by placing a piece of prepregfabric or sheet between two adjacent structural elements, especiallybetween the portion 40 of the outer platform 4, or the portion 30 of theinner platform 3, and the join regions 6 a, 6 b of the platforms andcores of the blades 2 a, 2 b.

Next, external plies of fresh material are draped over the pressure andsuction faces of the imidized cores of the blades 2 a, 2 b and over thefacing surfaces of the inner and outer platforms 3, 4. To do this, pliesof prepreg fabric 90, 91 precut to the correct dimensions are placed onspecific tools, as shown in FIG. 11.

A nonstick film is placed on the drape tools before the first plies areplaced thereon.

The drape tools are elements of a mold for the curing under highcompression of the sector 1 consisting of the separate imidized andassembled structural elements and of the fresh prepreg sheets placedbetween the various elements and covering those faces of the sector 1that have to define the stream of gas that will flow through the sector1 during operation of the compressor comprising an inlet guide vaneassembly produced from this type of sector.

During the step of imidizing the separate structural elements, most ofthe volatiles have been removed.

During the step of curing the sector 1, only the fresh prepreg sheetswill give off volatile gases at the start of the curing cycle. Sincethese “fresh” prepreg sheets cover a large area of imidized elements,which may be porous, the liquefied resin of the fresh prepreg sheets orfabrics will fill these pores.

The temperature-rise curves during curing depend on the resin used. Ifthis is a polyimide resin, as shown for example in FIG. 10, thetemperature is made to climb from 20° C. to 250° C. at a rate of 0.5°C./min, this 250° C. temperature is then maintained for 3 hours,followed by a further temperature rise from 250° to 320° C. at a rate of1° C./min. The temperature is maintained at 320° C. for 1 hour andfinally the temperature is raised to 360° C. at a rate of 0.5° C./minfollowed by a hold for 3 hours at 360° C. The cooling takes place in twophases having different cooling rates.

During the first cooling phase, the temperature is lowered by 0.5°C./min until a temperature of 220° C. is reached, while during thesecond cooling phase the temperature is lowered at a rate of 3° C./min.

At the start of the step of imidizing the fresh prepreg fabric plies,that is to say during the first phase of the curing step, the part isnot subjected to any compressive force.

At the end of imidizing the added layers, the part is increasinglycompressed so that it reaches a pressure level (for example 35 bar)sufficient to ensure good curing, when the temperature of the partreaches 310° C. This level of compression is maintained right to the endof cooling.

This very high level of compression makes it possible for the boundarywalls for the gas stream to have an excellent surface finish.

A relative vacuum of −50 mbar is maintained throughout the duration ofthe curing cycle.

1. A method of producing parts (1) made of a high-temperature-resistantcomposite, such as the vanes of an inlet guide vane assembly, saidcomposite comprising reinforcing fibers embedded in a heat-curedpolyimide resin matrix, said parts (1) having an inner platform (3), anouter platform (4) and at least one full blade (2 a, 2 b) connectingsaid platforms (3, 4), characterized by the following steps: a) theessential portions (30, 40) of the platforms (3, 4), of the blades (2 a,2 b) and of the blade/platform connection regions (6 a, 6 b) areproduced, as separate structural elements, by superposition or windingof the layers of prepreg fibers (reinforcing fibers preimpregnated withresin) with the exception of the external layers that have to form atleast the boundary wall for the stream of gases flowing through theguide vanes; b) said separate structural elements are imidized; c) saidseparate imidized structural elements are assembled; d) the externallayers of prepreg fibers are added in order to form the part; e) thepart thus formed is placed in a compression curing mold/contermoldassembly; and f) the part is cured by subjecting it to compressiveforces.
 2. The method as claimed in claim 1, characterized in that theexternal layers of fresh prepreg are added by draping the pressure sideof a blade, and the adjacent portions of the faces that face theplatforms, with first joining prepreg layers and by draping the suctionside of a blade, and the adjacent portions of the faces that face theplatforms, with second joining prepreg layers.
 3. The method as claimedin claim 1, characterized in that the structural elements are imidizedby heating them at 0.5° C./min with an intermediate hold for 120 minutesat 250° C. before cooling.
 4. The method as claimed in claim 3,characterized in that the structural elements are subjected to arelative vacuum of −50 mbar throughout the duration of the imidizationcycle.
 5. The method as claimed in claim 1, characterized in that thepart (1) is subjected to a compression of 35 bar when its temperaturereaches 310° C., and this compression is maintained until the end of thecooling.
 6. The method as claimed in claim 2, characterized in that thestructural elements are imidized by heating them at 0.5° C./min with anintermediate hold for 120 minutes at 250° C. before cooling.
 7. Themethod as claimed in claim 2, characterized in that the part (1) issubjected to a compression of 35 bar when its temperature reaches 310°C., and this compression is maintained until the end of the cooling. 8.The method as claimed in claim 3, characterized in that the part (1) issubjected to a compression of 35 bar when its temperature reaches 310°C., and this compression is maintained until the end of the cooling. 9.The method as claimed in claim 4, characterized in that the part (1) issubjected to a compression of 35 bar when its temperature reaches 310°C., and this compression is maintained until the end of the cooling.